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DS Download Play titles are available as installable CIAs. Games like Mario Kart DS demo and others like Sonic Colors are available in a light form on the DSiWare section of that ISO site.
Not mine, but this is as close as we've come to getting DS games on the 3DS as installable titles. They install to NAND, so if you're bored you can whip out the Mario Kart DS demo and play it with no card needed. Eh.

Some of the smaller DS games can be injected into DSiware titles already, injecting into these would give you the same thing. The problem with both ideas though is that the titles can only be installed to NAND, which isn't very big.